1. Field of the Invention
Aspects of the present invention relate to a piezoelectric transformer type high-voltage power source apparatus and an image forming apparatus including the same.
2. Description of the Related Art
In an image forming apparatus that forms an image in an electronic photographing process, a transfer unit contacts a photoreceptor to transfer the image from the photoreceptor to, for example, a print medium according to a direct transfer method. Specifically, the transfer unit is a conductive rubber in the form of a roller having a conductive body as a rotation axis. The driving of the transfer unit is controlled according to a process speed of the photoreceptor. Also, the polarity of a DC bias voltage, which is applied to the transfer unit, is the same as that of a transfer voltage of a corona discharge method.
Thus, in order to perform a reliable transfer using the transfer roller, a voltage of approximately 3 kV (required current is microamperes) is generally applied. According to a conventional technology, in order to generate a high voltage that is required for processing image formation, a coil-type electronic transformer has been used. However, the electronic transformer is formed with copper wire, bobbins, and magnetic cores, and when the electronic transformer is used to apply a voltage of approximately 3 kV, the output current value is very low. Accordingly, a leakage current in each part of the electronic transformer should be minimized. In order to minimize the leakage current, a method of making the coil of an electronic transformer with a mold made of an organic insulating material has been used. However, when this method is used, there are risks of producing smoke and fire, and a bigger electronic transformer in relation to the supplied power is required. Accordingly, it is difficult to reduce the size and weight of the electronic transformer.
In order to solve this problem, a method of generating a high voltage by using a slim-type light-weight high-output piezoelectric transformer has been considered. That is, if a piezoelectric transformer using a ceramic material is employed, it is possible to generate a high voltage with an efficiency greater than or equal to that of the electronic transformer. Furthermore, regardless of whether the primary side and the secondary side are combined, it is possible to space apart the primary side electrode and the secondary side electrode. Therefore, mold processing is not necessary for insulating and there are no longer any risks of producing smoke and fire. As a result, excellent properties that ensure a small-sized and light-weight high-voltage power source apparatus can be obtained.
As such, in a high-voltage power source apparatus using a piezoelectric transformer, a piezoelectric transformer controls an output by using a general frequency. However, in a frequency control performed by a high-voltage power source control circuit, problems (such as a variable width and inefficiency of an output voltage) exist. That is, in order to reduce an output voltage by increasing a frequency, if a frequency is changed too much, an output voltage increases at a next resonant point since a plurality of resonant points exist in a piezoelectric transformer. Accordingly, a variable width of an output voltage cannot be increased. In addition, frequency ranges resulting in high efficiency and low efficiency for generating a driving voltage exist in frequencies of a driving voltage. If a variable width of an output voltage increases, frequencies in a frequency range resulting in low efficiency should be used. Thus, the entire efficiency of the high-voltage power source apparatus is low.
In order to solve this problem, a technology to simultaneously control a frequency and a duty rate of a driving voltage has been suggested. When a frequency and a duty rate of a driving voltage are simultaneously controlled by using the above technology so as to make an output voltage uniform, a low output voltage and an increased variable width can be obtained. Accordingly, a constant voltage power source having excellent stability can be obtained. Also, when ranges resulting in high efficiency of a frequency and a duty rate are combined, efficiency of the high-voltage power source apparatus can be increased.
However, in the above technology, a control circuit to simultaneously control a frequency and a duty rate, which is included in a driving voltage control unit, generates a pyramidal wave by a charge/discharge circuit due to resistance and a capacitor. The frequency and the duty rate are simultaneously controlled based on the pyramidal wave. Accordingly, when a load current significantly increases due to a manufacturing irregularity of particular components and/or a change in temperature, the control circuit may exceed the resonant frequency such that control is impossible. In addition, the driving frequency cannot be used in the vicinity of a resonant frequency, and thus efficiency cannot be improved.
A conventional piezoelectric transformer type high-voltage power source apparatus will now be explained with reference to FIGS. 1 through 3. FIG. 1 is a block diagram to explain a conventional piezoelectric transformer type high-voltage power source apparatus.
Referring to FIG. 1, in the conventional piezoelectric transformer type high-voltage power source apparatus, a piezoelectric transformer T901 is ceramic. Diodes D902 and D903 and a high-voltage capacitor C904 rectify and smooth an alternating current (AC) output of the piezoelectric transformer T901 to form a constant voltage. The rectified and smoothed output voltage is provided to a transfer roller (not shown). Also, resistors R905, R906, and R907 divide the rectified and smoothed output voltage, before the rectified and smoothed output voltage is input to a non-inverted input terminal (+ terminal) of an operational amplifier Q909 through a protection resistor R908.
Meanwhile, an analog control signal Vcont of a high-voltage power source is input from a DC controller through a resistor R914 to an inverted input terminal (− terminal) of the operational amplifier Q909. The operational amplifier Q909, the resistor R914, and a capacitor C913 form an integrator circuit. The operational amplifier Q909 outputs a control signal (Vcont) that is integration-processed with an integration constant determined by the values of the resistor R914 and the capacitor C913.
The output end of the operational amplifier Q909 is connected to a Voltage-Controlled Oscillator (VCO) 910. Furthermore, the output end of the VCO 910 drives a transistor Q911 connected to an inductor L912, thereby providing a power source of a particular driving frequency to the primary side of the piezoelectric transformer T901. In this way, a high-voltage power source unit of an electronic photographing type image forming apparatus uses the piezoelectric transformer T901.
FIGS. 2 and 3 are waveform diagrams to explain a driving frequency of a piezoelectric transformer in a piezoelectric transformer type high-voltage power source apparatus according to a conventional technology. Referring to FIG. 2, a maximum output voltage of a piezoelectric transformer T901 occurs at a resonant frequency f0, and the output voltage decreases on either side of the resonant frequency f0. Accordingly, by controlling a driving frequency, the output voltage can be controlled. When the output voltage of the piezoelectric transformer T901 is to be increased, a driving frequency fx that is higher than the resonant frequency f0 is used as a new resonant frequency f0.
A high-voltage power source unit of an electronic photographing type image forming apparatus has a plurality of high-voltage power source circuits, and forms images with biasing outputs for charging, developing, and transferring. However, since the conventional piezoelectric transformer type high-voltage power source apparatus controls the driving frequency fx of the piezoelectric transformer T901 by processing an analog signal Vcont, as illustrated in FIG. 1, a time delay occurs before a desired output control voltage value is reached.
Also, a plurality of resonant points can exist in a piezoelectric transformer T901. For example, as illustrated in FIG. 3, 4 resonant points may exist in a piezoelectric transformer T901. As illustrated in FIG. 3, a first resonant point exists in which an output voltage of about 3.5 kV can be obtained if a driving voltage with a first resonant frequency f1 is applied. Furthermore, at the higher frequency side of the first resonant frequency f1 are a second resonant point (resonant frequency: f2) and a third resonant point (resonant frequency: f3) that maximize output voltages. Since each of the resonant points are a point at which an output voltage becomes a maximum (as illustrated in FIG. 3), if the frequency of a driving voltage is changed to a frequency either higher or lower than a resonant frequency, the output voltage decreases.
However, even when the frequency is changed from the resonant frequency, if the maximum value of an output voltage is set to kilovolts, the frequency of the output voltage does not reduce to hundreds of volts or less. This is because, if the frequency is changed greatly, the frequency does not converge on 0 as the frequency approaches a next resonant frequency. Therefore, after a certain minimum frequency is reached, the output voltage increases until the next resonant frequency is reached.
In the frequency range of a driving voltage, a range exists within which an output voltage of the piezoelectric transformer T901 may be most efficiently obtained (such as a range in the vicinity of a resonant frequency). However, in order to increase the variable width of an output voltage range, a frequency in a range within which poor efficiency results must also be used. Thus, the efficiency as a whole is lowered.